A number consists of two digits. The sum of the digit is 10. On reversing the digits of the number, the number decreases by 36. What is the product of the two digits?

Let the unit digits of the number be N and 10s digit be M
∴ Number = 10M + N
According to the question
MN = 10 .......(i)
And 10N + M = (10M + N) - 36
⇒ 10N + M - 10M - N = -36
⇒ 9N - 9M = - 36
⇒ N - M = -4 .......(2)
On Adding eq. (i) and (ii) we get
N + M = 10
N - M = - 4
2N = 6
∴ N = 3 and M = 7
∴ Required product of two digits = 3 x 7 = 21